1. Field of the Invention
The present invention relates to a crystal oscillator, and more particularly, to a crystal oscillator preventing abnormal oscillation by which an oscillation frequency suddenly (discontinuously) changes due to a temperature change.
2. Description of the Related Art
A crystal oscillator is used as a frequency and time reference source in various types of electronic devices such as a digital control device, a communications device, etc. Among communication devices where a crystal oscillator is used, a communications device that is normally installed outdoors and demanded to be used under an environment of a severe temperature change, for example, like a broadband wireless communications device which secures a large amount of a communications line.
FIGS. 1 and 2 exemplify a general crystal oscillator. FIG. 1 is a circuit diagram showing the crystal oscillator, whereas FIG. 2 is a cross-sectional view of the structure of the crystal oscillator.
The crystal oscillator is mainly composed of a crystal vibrator 1 and an oscillation circuit unit 2, and forms, for example, an oscillation circuit 3 of a Colpitts type. The crystal vibrator 1 has a configuration where an electrode (not shown) is formed, for example, in an AT-cut quartz crystal element, and the quartz crystal element is held and hermetically sealed in a metal box 5 from which a lead 4 is extended.
The oscillation circuit unit 2 is composed of a capacitor that forms a resonance circuit along with the crystal vibrator 1 (inductor component), an amplifier for oscillation, etc. Normally, an oscillation amplifying stage 6 amplifying the oscillation output from the oscillation circuit unit 2 is connected to the oscillation circuit unit 2. The crystal vibrator 1 and other elements 7, which form the crystal oscillator, are arranged on a circuit board 9 having a lead 8, and sealed by a cover not shown.
With such a crystal oscillator, the oscillation frequency of an output signal changes with temperature mainly due to the frequency-temperature characteristic of the crystal vibrator 1.
If the quartz crystal element of the crystal vibrator 1 is AT-cut, the frequency-temperature characteristic of an oscillation frequency becomes a tertiary curve whose inflection point is close to an ordinary temperature (25xc2x0 C.). Normally, as shown in FIG. 3, a crystal vibrator is selected so that the frequency-temperature characteristic that has a maximum value in a low-temperature area equal to or lower than an ordinary temperature, and a minimum value in a high-temperature area higher than the ordinary temperature is implemented. Namely, the crystal vibrator (the cutting angle of the AT cutting) is selected to have such a frequency-temperature characteristic, whereby an oscillation frequency change due to a temperature change is suppressed within a monotonously changing range between the maximum and the minimum values in a wide temperature range having the ordinary temperature as a center. Note that the temperature points of the maximum and the minimum values depend on the cutting angle of the AT cutting.
However, the crystal oscillator having the above described configuration has a problem of causing abnormal oscillation in a low-temperature area of 0xc2x0 C. or lower. The abnormal oscillation referred to here means not a phenomenon that an oscillation frequency moderately changes according to the frequency-temperature characteristic, but a phenomenon that the oscillation frequency discontinuously changes, and is commonly called a jump (micro-jump) of an oscillation frequency.
FIG. 4 shows such a jump (micro-jump) in the frequency-temperature characteristic of the crystal vibrator.
The frequency-temperature characteristic has an extremely small resonance point at a certain temperature point equal to or lower than 0xc2x0 C. due to some reason or coincidence of a plurality of conditions, so that an oscillation frequency suddenly changes, for example, as shown in FIG. 4. This is also called a micro-jump, and is not found when the frequency-temperature characteristic is measured in a manufacturing process. For this reason, there is a problem that abnormal oscillation in a low-temperature area cannot be avoided under an environment of 0xc2x0 C. or lower. Note that, however, the micro-jump does not occur in all of crystal oscillators.
The above provided example refers to a mere crystal oscillator. However, abnormal oscillation occurs similarly, for example, in a crystal oscillator having a configuration where a compensation voltage is applied by inserting a voltage variable capacity element, which is not shown, in an oscillation closed loop of the crystal oscillator, or in a crystal oscillator of, what is called, a temperature compensation type, which compensates for and flattens the frequency-temperature characteristic by inserting a temperature compensation circuit composed of a parallel circuit of a thermistor and a capacitor in an oscillation closed loop.
Namely, even in these crystal oscillators, the crystal vibrator 1 itself depends on an ambient temperature, and exhibits the frequency-temperature characteristic having a resonance point in a low-temperature area. Therefore, a sudden change cannot be avoided although temperature compensation is made, and abnormal oscillation occurs. As a factor of causing a micro-jump, there are various theories such as a result of a phenomenon that condensation occurs at a temperature equal to or lower than 0xc2x0 C. on the surface of a quartz crystal element within a sealed box, and the like. However, its details are not elucidated at the present moment.
The present invention aims at providing a crystal oscillator preventing abnormal oscillation in a low-temperature area.
A crystal oscillator according to the present invention comprises an oscillation unit and a heat source unit.
The oscillation unit is composed of a crystal vibrator having a frequency-temperature characteristic with which a resonance frequency changes according to a temperature, and an oscillation circuit unit.
The heat source unit, which abuts against the crystal vibrator, keeps the temperature of the crystal vibrator higher than a temperature at which the crystal vibrator causes abnormal oscillation.
A crystal oscillator having another configuration of the present invention comprises an oscillation unit having a crystal vibrator, and a heat source unit keeping the temperature of the crystal vibrator higher than a temperature where the crystal vibrator causes abnormal oscillation.
This abnormal oscillation is caused, for example, by a micro-jump that occurs in the crystal vibrator.
The crystal vibrator is kept, for example, at a temperature higher than 0xc2x0 C.
The heat source unit is configured, for example, by a power transistor that amplifies an oscillation output.
According to the present invention, the crystal vibrator is kept by the heat source unit at a temperature higher than a particular temperature, so that abnormal oscillation does not occur even in a low-temperature